1. Field of the Invention
The present invention relates to an image exposing apparatus including a beam scanning means for scanning a beam emitted from a semiconductor laser device to photosensitive material and laser driving means for driving the semiconductor laser device, the laser driving means being operable to vary its driving current to the semiconductor laser device in accordance with a received image signal.
2. Description of the Related Art
Such image exposing apparatus of the above-noted type is used for scanning photosensitive material with optical beam thereby to form a desired image on the photosensitive material.
The apparatus employs a semiconductor laser device for generating the optical beam. As the beam outputted from the semiconductor laser device can be modulated directly by its driving current, any desired image can be formed on the photosensitive material by varying the driving current for the semiconductor laser device according to an image signal inputted thereto.
Therefore, in order to expose photosensitive material with a desired exposure amount, it is necessary to control the beam output from the semiconductor laser device. As a method of controlling this, the conventional art noted fact that an oscillation threshold current of the semiconductor laser device varies with time and hence proposes a construction wherein the oscillation threshold current of the semiconductor laser device is measured at an appropriate timing and then to this measured oscillation threshold current value, a current value corresponding to a target beam output is added, thereby to obtain a driving current to be applied to the semiconductor laser device.
However, when the above-described conventional construction was applied to the device for exposure-forming an image through direct modulation of the beam output of the semiconductor laser device, there was often observed a phenomenon that the resultant exposed image would exhibit disadvantageous change in its “coloring” with time. In this respect, there has been a need for improvement.
The present invention has been made in view of the above-described state of the art. The principal object of the invention is to ensure, for an extended period of time, good quality in images to be obtained on the photosensitive material by the technique of exposing and forming an image through direct modulation of the semiconductor laser device.